python3 default class attribute to method in same class

Question

class Test:
    func = self.func2 # default: func is func2 by default
    # tried Test.func2 as well and Pycharm shows error

    def __init__(self, func=None):
        if func is not None:
            self.func = func

    def func1(self):
        pass

    def func2(self):
        pass

Can anyone advise how to achieve something like the above?

I have tried setting the func parameter to default to func2 in the constructor too but this also errors.

so later, somewhere in the class Test, I can just call self.func instead of repeatedly running conditions to find out whether func1 or func2 should be used

self.func()

**** SOLUTION (what I did) ****

in the main.py:

t = Test(func=Test.func1)
t.func(t, **kwargs)

this should successfully call t.func1 (or t.func2 if specified) with access to the t instance_attributes through a call of higher order method t.func

default an attribute to one of many alternative implementations of the same method for reuse in the class — Loveen Dyall
– Loveen Dyall, Commented May 27, 2019 at 15:34
Should the func passed to __init__ be either func1 or func2? — Tianbo Ji
– Tianbo Ji, Commented May 27, 2019 at 15:41

Reblochon Masque · Accepted Answer · 2019-07-20 00:39:48Z

1

You could set the default in the __init__ method, based on the value of the func parameter:

Maybe something like this:

class Test:

    def __init__(self, func=None):
        if func is None:
            self.func = self.func2
        else:
            self.func = func

    def func1():
        print('func1')

    def func2(self):
        print('func2')

Test().func()
Test(Test.func1).func()

output:

func2
func1

edited Jul 20, 2019 at 0:39

answered May 27, 2019 at 15:40

Reblochon Masque

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5 Comments

Loveen Dyall Over a year ago

question, how do i pass Test.func2 as the func parameter on construction and not re assign after construction?

Reblochon Masque Over a year ago

You can do t = Test(func=Test.func2)

Loveen Dyall Over a year ago

It says something about missing 1 positional argument. I'm guessing this is due to a lack of a 'self' first parameter on an instance method?

Loveen Dyall Over a year ago

solved it by passing the target object instance as the first parameter to higher order method defined in the target (e.g. Test instance). hence the target object (Test instance) resolves 'self'

Reblochon Masque Over a year ago

actually, self was missing as param for func2, sorry.

Blckknght · Accepted Answer · 2019-07-20 00:50:57Z

Your code doing an assignment at class scope would work if you don't use self (which isn't defined) and if you move it to the end of the class definitions, so that func2 will exist in the namespace when it runs.

class Test:
    def __init__(self, func=None):
        if func is not None:
            self.func = func

    def func1():
        pass

    def func2():
        pass

    func = func2 # this will work down here

I'd note that when you assign a function to a class variable, it's going to be treated like a method, so you might need to change func2 to expect self as its first argument. The binding behavior won't happen for methods assigned as instance variables, like any func passed in to the __init__ method. If you want binding behavior for those, you'd need to implement it yourself (perhaps with functools.partial).

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